1. Isotopes
Atoms with SAME number of PROTONS (atomic number) but DIFFERENT numbers of neutrons (i.e. mass number)
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons 12 13 14

2. Notation 6 C 6 C 6 C 12 13 14
Carbon-12
Carbon-13
Carbon-14

3. Relative atomic mass
Is the AVERAGE mass of all of the isotopes of an element, taking into account their abundance in nature.
EXAMPLE
If there are two isotopes of an element, one with a mass of 12 and one with a mass of 14, and each is equally common in nature, the RELATIVE ATOMIC MASS of that element is 13.
If there are two isotopes of an element, one with a mass of 12 and one with a mass of 14, but the first makes up 25% of the element as found in nature and the second makes up 75% , the RELATIVE ATOMIC MASS of that element is 13.5.